1. Field of the Invention
The present invention relates generally to digital communications. More specifically, the invention relates to a system for and method of using a code division multiple access air interface which greatly reduces the signal power required for the global and assigned-pilots while improving performance by using the quadrature phase shift keyed (QPSK) traffic signal for a particular channel to perform channel estimation and carrier recovery.
2. Description of the Prior Art
Most advanced communication technology today makes use of digital spread spectrum modulation or code divisional multiple access (CDMA). Digital spread spectrum is a communication technique in which data is transmitted with a broadened band (spread spectrum) by modulating the data to be transmitted with a pseudo-noise signal. CDMA can transmit data without being affected by signal distortion or an interfering frequency in the transmission path.
Shown in FIG. 1 is a simplified CDMA communication system that involves a single communication channel of a given bandwidth which is mixed by a spreading code which repeats a predetermined pattern generated by a pseudo-noise (pn) sequence generator. A data signal is modulated with the pn sequence producing a digital spread spectrum signal. A carrier signal is then modulated with the digital spread spectrum signal establishing a forward link, and transmitted. A receiver demodulates the transmission extracting the digital spread spectrum signal. The transmitted data is reproduced after correlation with the matching pn sequence. The same process is repeated to establish a reverse link.
During terrestrial communication, a transmitted signal is disturbed by reflection due to varying terrain and environmental conditions and man-made obstructions. This produces a plurality of received signals with differing time delays at the receiver. This effect is commonly known as multipath propagation. Moreover, each path arrives delayed at the receiver with a unique amplitude and carrier phase.
To identify the multiple components in the multipath propagation, the relative delays and amplitudes and phases must be determined. This determination can be performed with a modulated data signal, but typically, a more precise rendering is obtained when compared to an unmodulated signal. In most digital spread spectrum systems, it is more effective to use an unmodulated pilot signal discrete from the transmitted modulated data by assigning the pilot an individual pn sequence. A global-pilot signal is most valuable on systems where many signals are transmitted from a base station to multiple users.
In the case of a base station which is transmitting many channels, the global-pilot signal provides the same pilot sequence to the plurality of users serviced by that particular base station and is used for the initial acquisition of an individual user and for the user to obtain channel-estimates for coherent reception and for the combining of the multipath components. However, at the required signal strength, the global-pilot signal may use up to 10 percent of the forward direction air capacity.
Similar multipath distortion affects a user's reverse link transmission to the base station. Inserting in each individual user's return signal an assigned-pilot may consume up to 20 percent of the total reverse channels air capacity.
Without phase and amplitude estimation, noncoherent or differentially coherent reception techniques must be performed. Accordingly, there exists a need for a coherent demodulation system that reduces the air capacity of the global-pilot and assigned-pilot signals while maintaining the desired air-interface performance.